a. Field of the Invention
The present invention relates generally to the removal of components from decommissioned nuclear power plants and similar facilities, and, more particularly, to the filling of decommissioned nuclear reactor vessels with cellular cement grout prior to transportation to a depository or other disposal site.
b. Related Art
The decommissioning of nuclear power plants and other nuclear facilities normally involves removing the reactor components from the facility and transporting them to a long-term containment area, typically in an arid and remote region.
Chief of these components is the reactor vessel itself. Conventionally, the reactor vessel comprises a tall (e.g., 40-60 feet), generally cylindrical steel shell. The core, which somewhat resembles the tubing of a boiler, is typically located towards the bottom of the shell, with various inlet and outlet pipes being provided in the upper parts of the vessel. During normal operation, the reactor vessel is kept filled with water, with the hot outflow being employed to power turbines or other generation equipment.
The reactor vessel is normally housed in a reactor building, a prominent, frequently domed structure that is designed to contain contamination from the vessel in the event of catastrophic failure. During decommissioning, the reactor vessel must be filled with cement prior to removal from the reactor building. The cement stabilizes the core and other internal components of the vessel for movement and transportation, during which the vessel will normally be positioned on its side on a rail car, barge or other vehicle. The cement also provides shielding around the radioactive core in place of the original water, significantly reducing exposure doses of personnel engaged and preparing the vessel for transport.
A difficulty can arise, however, when removing the cement-filled vessel from the reactor building. Reactor buildings are almost universally provided with installed crane systems that were used to install the reactor vessels when the facilities were first built, and for use in subsequent maintenance activities. As a rule, however, the original designs did not fully anticipate current decommissioning procedures and requirements. The installed cranes are consequently capable of lifting the reactor vessels when empty, but, within safe limits, are unable to lift (commonly referred to as “picking”) the vessels if full of heavy cement grout.
An effective solution has been found in the use of lightweight cellular cement grouts. As is known to those skilled in the relevant art, such grouts generally employ a mixture of hydraulic cement (typically, ordinary Portland cement) and a cellular foam material. The cellular cement grout material has a highly fluid consistency and is able to flow throughout the interior of the vessel and into the various chambers and conduits, and when cured forms a rigid mass that effectively stabilizes the internal components for removal and transportation. The water in the cellular cement grout, which remains in the cement matrix when hydrated, provides a shielding effect that helps substitute for the water that filled the vessel when in operation, as do certain mineral constituents of the cement. The cellular foam, in turn, forms pockets of air (or other gasses) within the matrix and therefore reduces the overall weight of the fill material to a level where the vessel can still be “picked” and removed using the installed crane
Although cellular cement grout thus provides an optimal fill material, its use is complicated by the fact that all water must be removed before filling the vessel with grout. If the cellular grout encounters significant amounts of water within the vessel an excessively high water-to-cement ratio will be created at the interface, so that the cellular structure of the grout will tend to dissipate and collapse in a progressive fashion. As this happens, more and more grout is needed to fill the reactor vessel and overall density quickly rises, potentially to the point where the safe weight limit is exceeded.
Removing all water from the vessel prior to grouting, however, involves considerable difficulty and expense. Although the bulk of the water can be removed using installed piping and suction hoses, a significant amount (typically, approximately 200 gallons) remains trapped and virtually inaccessible in the bottom of the vessel, primarily beneath a baffle structure that is usually installed near the bottom of the vessel beneath the core. This area can be reached, if at all, only through very small (e.g., 1-inch) tubes and openings. Drilling a hole through the bottom of the shell to drain the residual water is generally impractical; the steel of the shell is very thick (e.g., 8-inches) and tough and the stay time necessary to drill through it would be excessive, particularly in view of the relative lack of shielding as the vessel becomes emptied of water. Furthermore, the water and residual radioactive materials in the bottom of the shell present serious safety hazards, should these happen to escape from the hole and expose personnel during the drilling operation. In this regard, it should be noted that strict limits have been set for lifetime radiation exposure, and it is therefore critical that exposures be minimized lest it become necessary to retire and replace trained personnel.
Consequently, even with the most diligent of efforts, a minimum of 4-12 inches of residual, contaminated water usually remains in the bottom of the vessel, creating an ever-present threat to the success of the grouting operation.
Accordingly, there exists a need for a method for filling a decommissioned nuclear reactor vessel with cellular cement grout while accommodating the presence of residual water in the bottom of the vessel. Furthermore, there exists a need for such a method that produces a fill having a total density sufficiently low that the reactor vessel can be picked and removed using the installed crane system of the reactor building. Still further, there exists a need for such a method that obviates the possibility of the cellular structure of the grout dissipating or collapsing during the filling operation. Still further, there exists a need for such a method that uses economical materials/equipment and that can be performed in an efficient manner with minimal exposure of operating personnel.